The present invention relates to the removal of polychlorinated dibenzo-p-dioxins, (referred to hereinafter as "PCDD's" or simply "dioxins"), and polychlorinated dibenzofurans, (referred to hereinafter as "PCDF's") from paper mill sludge by supercritical water oxidation.
As used herein, paper mill sludge refers to that portion of the papermaking process waste stream that contains 1) fibrous fines and debris composed of carbohydrates such as celluloses and hemicelluloses, 2) inorganic materials such as aluminum silicates (clays), fillers, coatings, and the like, and 3) small amounts of residual pulping chemicals. Such sludges can include tissue mill sludge, pulp mill sludge, sludge from the de-inking of secondary fibers, and the like.
Efficient management of solid waste streams, of which industrial wastes such as paper mill sludge constitute a significant part, has become an important societal theme. Until recently, landfilling has been the disposal method of choice for solid wastes such as paper mill sludge due to economic appeal. However, recent legislative trends indicate that in the future landfills may become harder to obtain and less economical to maintain. Increased attention has thus begun to be placed on alternative methods of waste disposal such as recycling, incineration, and waste reduction at the source.
In the case of paper mill sludge disposal, conventional incineration is in general a feasible alternative to landfilling. However, in recent years, a number of concerns have arisen about the incineration process as applied to this type of sludge.
These concerns relate to the presence of small quantities of reportedly toxic compounds, specifically, PCDD's and PCDF's, in paper mill sludge. It has been reported in the literature that conventional incineration may not always destroy these chloro-organic compounds and, in fact, under certain circumstances, incineration can generate more of these compounds unless expensive after-burners are used. (See Chlorinated Dioxins and Dibenzofurans in the Total Environment II, Keith, L. H., Rappe, C. and Choudhary, G., eds., Butterworth Publishers, Boston, Massachusetts, 1985, pages 48-49.) Moreover, even when after-burners are used, cold spots in the burners sometimes occur inadvertently, resulting in discharges of PCDD's and PCDF's into the atmosphere.
Kraft pulps, when bleached with sequences including an elemental chlorine stage, sometimes contain small but detectable levels of PCDD's and PCDF's. Recycled, bleached Kraft fibers under a variety of guises (e.g., coated paper, ledger paper, etc.) are often present in substantial quantities in waste paper stock as purchased from commercial dealers. Hence waste paper stock, intended for recycling, i.e., the production of secondary fibers, may contain small but detectable amounts of PCDD's and PCDF's.
The processing steps currently used to treat recycled waste paper (e.g., pulping/screening/flotation/bleaching) are not effective in removing PCDD's and PCDF's from stock containing chlorine-bleached fibers. Therefore, paper mill sludges generated from the secondary fiber industry, and from any pulping process (e.g. Kraft) which utilizes one or more elemental chlorine bleaching stages, may contain PCDD's and PCDF's.
PCDD's and PCDF's are large groups of chloro-organic compounds which have become ubiquitous in industrial societies. The structures of these compounds are as follows, where in each case x+y=1-8: ##STR1##
Of the various possible isomers of these compounds, the following are reportedly the most toxic:
2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) PA0 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD) PA0 2,3,7,8-tetrachlorodibenzofuran (TCDF) PA0 1,2,3,7,8-pentachlorodibenzofuran (PCDF) PA0 2,3,4,7,8-pentachlorodibenzofuran (PCDF). PA0 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (HCDD) PA0 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin (HCDD) PA0 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin (HCDD) PA0 1,2,3,6,7,8-hexachlorodibenzofuran (HCDF) PA0 1,2,3,7,8,9-hexachlorodibenzofuran (HCDF) PA0 1,2,3,4,7,8-hexachlorodibenzofuran (HCDF) PA0 2,3,4,6,7,8-hexachlorodibenzofuran (HCDF) PA0 2,3,7,8-tetrachlorodibenzo-p-dioxin PA0 1,2,3,7,8-pentachlorodibenzo-p-dioxin PA0 2,3,7,8-tetrachlorodibenzofuran PA0 1,2,3,7,8-pentachlorodibenzofuran PA0 2,3,4,7,8-pentachlorodibenzofuran PA0 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin PA0 1,2,3,7,8,9-hexachlorodibenzo-p-dioxin PA0 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin PA0 1,2,3,6,7,8-hexachlorodibenzofuran PA0 1,2,3,7,8,9-hexachlorodibenzofuran PA0 1,2,3,4,7,8-hexachlorodibenzofuran PA0 2,3,4,6,7,8-hexachlorodibenzofuran.
Also reported to be highly toxic are:
See Rappe et al., "Analysis of Polychlorinated Dibenzofurans and Dioxins in Ecological Samples," in Chlorinated Dioxins and Dibenzofurans in the Total Environment II, Keith, L. H., Rappe, C. and Choudhary, G., eds., Butterworth Publishers, Boston, Massachusetts, 1985, pages 125-126.
In the recent past, the issue of health hazards associated with PCDD's and PCDF's has received much attention in the news media. PCDD's and PCDF's are known to cause a temporary form of a skin ailment known as "chlor-acne." Also, PCDD's and PCDF's (particularly 2,3,7,8-TCDD) have proved to be extremely toxic to certain animals in laboratory studies, in particular to guinea pigs (LD.sub.50=0.6-4.0 micrograms/kilogram). See, for example, Ottoboni, A., The Dose Makes the Poison, Vincente Books, Berkeley, California, 1984, and Dioxins, NTIS Report No. PB82-136847, Industrial Environmental Research Laboratory, Cincinnati, Ohio, November, 1980, Section 6.
Because of this reported high level of toxicity to a common laboratory test animal (i.e., the guinea pig), there is a general concern as to the long-term effects of PCDD's and PCDF's on human physiology. Accordingly, there is an important need to remove or substantially reduce the content of PCDD's and PCDF's from paper mill sludge as part of the disposal process. It is an object of the present invention to respond to this need.
Some references exist in the literature regarding attempts to decompose or destroy dioxins in a state of solution in liquid media (e.g., hexane) or in substances such as silica gel or clay via photolytic techniques, e.g., UV radiation. (See Ottoboni, supra; Crosby, D.G., et al., Science, Vol. 173, Aug. 20, 1971, pages 173-174; Plimmer, J. R., Bull. Environm. Contam. Toxicol., Vol. 20, 1978, pages 87-92; Botre, Claudio, Adriana Memoil, and Franco Alhaique, Environmental Science and Technol., Vol. 12, No. 3, March 1978, pages 335-336; Crosby, D. G., et. al., Environmental Health Perspectives, Sept. 1973, pages 259-266; Dulin, David, Howard Drossman, and Theodore Mill, Environ. Sci. Technol., Vol. 20, No. 1, 1986, pages 72-77; and Podoll, R. Thomas, Helen M. Jaber, and Theodore Mill, Environ. Sci. Technol., Vol. 20, No. 5, 1986, pages 490-492.)
The process has been shown to work to an extent but appears to be highly dependent upon the presence of a hydrogen donor solvent, the type and level of impurities present, and the substrate. Furthermore, the photoproduct resulting from irradiation of 2,3,7,8-TCDD has been reported to be trichloro- and dichloro-benzo-p-dioxins, which are less toxic than 2,3,7,8-TCDD but, nevertheless, are undesirable. No attempt has been made to explore the potential of the photolytic technique of destroying PCDD's and PCDF's in cellulosic materials such as paper mill sludge
U.S. Pat. Nos. 4,338,199 and 4,543,190 to Modell describe a process in which organic materials are oxidized in supercritical water. The '199 patent includes a general statement that its process can be used to remove toxic chemicals from the wastes generated by a variety of industries including forest product wastes and paper and pulp mill wastes. No specific mention is made of dioxins. The '190 patent describes the treatment of various chlorinated organics other than dioxins with supercritical water and states that conversion of these materials to chlorinated dibenzo-p-dioxins was not observed (see Example 6).
Although both patents mention paper mill sludge as one of the materials which can be used in the supercritical water process, neither patent discloses work actually done on this type of sludge. In particular, neither patent discloses or suggests that supercritical water oxidation can remove PCDD's and PCDF's from paper mill sludge.
The use of supercritical water to treat organic waste materials is also disclosed in PCT Patent Publication No. WO 81/00854, Modell et al., U.S. Pat. No. 4,113,446, Burleson, U.S. Pat. No. 4,564,458, and Titmas, U.S. Pat. No. 4,594,164. The treatment of wood chips and black liquor from pulping with supercritical water or near supercritical water is described in Modell, PCT Patent Publication No. WO 81/00855. See also Modell, M., "Gasification and Liquefaction of Forest Products in Supercritical Water", Fundam. Thermochem. Biomass Convers., 1985, pp. 95-119; and West et al., "Pyrolysis of 1,3-butanediol as a model reaction for wood liquefaction in supercritical water", Can. J. Chem. Eng., 1987, vol. 65, pp. 645-650.
A summary of experiments performed by Modar, Inc., using the Modell supercritical water process was published in Chemosphere--Chlorinated Dioxins and Related Compounds 1987, McNelis et al., editors, Pergamon Press, New York, Vol. 18, Nos. 1-6, 1989, page 50. As described therein, bench-scale tests were performed on soils and liquid wastes contaminated with chlorobenzenes and PCDDs. Supercritical water oxidation was found to remove 2,3,7,8-TCDD and chlorobenzenes from soil and to remove 2,3,7,8-TCDD, TCDD's and OCDD from liquid wastes.
In considering the Modell work with super-critical water, it is important to keep in mind the differences between different types of wastes and the properties of PCDD's and PCDF's. In particular, supercritical water experiments performed on municipal wastes, specifically, municipal sludge, cannot be used to predict whether or not PCDD's and PCDF's can be oxidized under supercritical conditions in paper mill sludge.
This is so because of the difference in composition of municipal sludge and paper mill sludge. Thus, paper mill sludge contains lime mud, clay, fillers, relatively large amounts of cellulose fibers, and relatively low amounts of grease and fats. Additionally, municipal sludge contains substantial amounts (up to about 40%) of proteins. Also, compounds of potassium and phosphorous are often present in significant quantities in municipal sludges, whereas paper mill sludges, in general, contain much smaller amounts of such compounds.
These compositional differences make predictions from one type of sludge to another unreliable especially when the binding properties of PCDD's and PCDF's are taken into account. Thus, as known in the art, high surface area materials, such as cellulose fibers and/or clay, are capable of strongly binding organic compounds such as PCDD's and PCDF's. See Srinivasan et al., "Binding of OCDD, 2,3,7,8-TCDD and HCB to Clay-Based Sorbents," in Chlorinated Dioxins and Dibenzofurans in Perspective, Rappe, C., Choudhary, G., and Keith, L. H., eds., Lewis Publishers, Inc., Chelsea, Michigan, 1986, page 532.
Moreover, in a water system, PCDD's and PCDF's will adhere to solid sorbents rather than remaining free in solution. Thus, partition coefficients in the range of 2.8-67.1.times.10.sup.3 have been reported for 2,3,7,8-TCDD for a variety of sorbents including hydroxy aluminum-clay and activated carbon. This compound also adheres quite well to glass in a water environment. See Srinivasan et al., supra at pages 531-537.
These binding properties in combination with the fiber, mud and filler composition of paper mill sludge makes the effective removal of PCDD's and PCDF's from this type of sludge particularly difficult to achieve. It is to this challenge that the present invention is directed.
In addition to the foregoing specific references, a general review of the use of supercritical fluids in various industrial and pollution control processes can be found in Eckert et al., "Supercritical fluid processing", Environ. Sci. Technol., 1986, vol. 20, pp. 319-325. Among other things, these authors discuss a study in which supercritical ethylene was used to remove trichlorophenol from from soil as a model for the removal of dioxins and polychlorinated biphenyls (PCBs). No data are presented for dioxins.
Along these same lines, Pang et al., "Supercritical Extraction of Aromatic Hydrocarbon Solids and Tar and Bitumens", Ind. Eng. Chem. Process. Des. Dev., 1985, vol. 24, pp. 1027-1032 discuss the use of various supercritical fluids to extract organic materials from tar sands. The reference mentions the possibility of using supercritical extraction to remove hazardous materials such as PCBs and dioxin from soils.
Other studies involving the use of supercritical fluids to remove hazardous organic materials from environmental solids such as soil can be found in Groves et al. "State-of-the-art on the supercritical extraction of organics from hazardous wastes", CRC Critical Reviews in Environmental Control, 1985, vol. 15, pp. 237-274; Hawthorne et al., "Extraction and Recovery of Polycyclic Aromatic Hydrocarbons from Environmental Solids Using Supercritical Fluids", Anal. Chem., 1987, vol. 59, pp. 1705-1708; Dooley et al., "Supercritical Fluid Extraction and Catalytic Oxidation of Toxic Organics from Soils", EPA Report No. 600/9-87/018F, pp. 383-397; and Brady et al. "Supercritical Extraction of Toxic Organics from Soils", Ind. Eng. Chem. Res., 1987, vol. 26, pp. 261-268.
Various uses of supercritical fluids in the processing of materials have been disclosed in the literature. For example, supercritical carbon dioxide has been used to remove tall oil and turpentine from coniferous woods in Fremont, U.S. Pat. No. 4,308,200, to extract lignin from the black liquor produced by the Kraft process for pulp production in Avedesian, U.S. Pat. No. 4,493,797, to treat refinery sludges in European Patent Publication No. 314,223, to regenerate absorbents used in waste water treatment systems in Modell, U.S. Pat. Nos. 4,061,566 and 4,147,624, to sterilize pharmaceuticals in Pilz et al., U.S. Pat. No. 4,263,253, to remove off-flavor materials from textured vegetable products in Sevenants, U.S. Pat. No. 4,675,198, to remove gamma-linolenic acid from fruit seeds in Traitler et al., U.S. Pat. No. 4,703,060, and to decaffeinate coffee in Katz, U.S. Pat. No. 4,472,442; Toro et al., U.S. Pat. No. 4,728,525 and Kaleda et al., U.S. Pat. No. 4,767,634. See also, Friedrich, U.S. Pat. No. 4,466,923; Lawson et al., U.S. Pat. No. 4,495,095; Myerson, U.S. Pat. No. 4,550,198; Panzner et al., U.S. Pat. No. 4,554,170; Japikse et al., U.S. Pat. No. 4,647,466; Ritter and Campbell, "The Effects of Supercritical Carbon Dioxide Extraction on Pine Wood Structure", Biotechnology and Bioengineering Symp., 1986, no. 17, pp. 179-182; Hatakeda et al., "Extraction of Sugi (Cryptomeria japonica D. Don) with supercritical carbon dioxide", Nipon Kagaku Kaishi, 1987, no. 5, pp. 931-933; Shishikura et al., "Concentration of Tocopherols from Soybean Sludge by Supercritical Fluid Extraction", J. Jpn. Oil Chem. Soc., 1988, vol. 37, pp. 8-12; and Li and Kiran "Interaction of Supercritical Fluids with Lignocellulosic Materials", Ind. Eng. Chem. Res., 1988, vol. 27, pp. 1301-1312.
In addition to their use in waste treatment and materials processing, supercritical fluids have been used in connection with various analytic procedures. For example, Suprex Publication No. TN-022, Suprex Corporation, Pittsburgh, PA, 1989, mentions the use of supercritical carbon dioxide as part of an analytical procedure for assaying dioxins. Similarly, Hawthorne et al., "Directly coupled supercritical fluid extraction-gas chromatographic analysis of polycyclic aromatic hydrocarbons and polychlorinated biphenyls from environmental solids", J. Chromatogr., 1987, vol. 403, pp. 63-76, discuss the use of supercritical fluid extraction coupled to a gas chromatograph to analyze environmental solids, e.g., urban dust, for organic pollutants, specifically, polycyclic aromatic hydrocarbons. The extraction was performed using nitrous oxide as the supercritical fluid. Along similar lines, Schneiderman et al., "Determination of anthraquinone in paper and wood using supercritical fluid extraction and high-performance liquid chromatography with electrochemical detection", J. Chromatogr., 1987, vol. 409, pp. 343-353, describe the combination of supercritical fluid extraction using carbon dioxide, high-performance liquid chromatography, and electrochemical detection to analyze Kraft paper and pine plywood sawdust for anthraquinone.
Significantly, none of these references in any way discloses or suggests that PCDD's and PCDF's can be effectively removed from paper mill sludge via oxidation with supercritical water.